Wednesday, December 12, 2012

Diatoms: Our Friend, Our Foe

by EBJ

When most
people think of photosynthesis they think of lush, green plants and trees, but
that is only part of the story. One fifth of all photosynthesis on Earth is
performed by a type of oceanic plankton called diatoms (1). Diatoms are
unicellular brown algae that are unique in that they form their cell walls out
of silica. Most of the carbon they fix feeds directly into the oceanic food
chain; the remaining fraction is sequestered when it falls to the deep sea
floor (1). Despite the vital role diatoms play in the global carbon cycle, a
few species can cause life-threatening illness.

A
diatom bloom off Vancouver Island (2)

Seafood
causes one fifth of cases of food-borne illness in the United States, and half
of these are due to toxins produced by algae (3). Some chemicals, such as these
algal toxins, accumulate in the tissues of their predators, and become even
more concentrated in the next predators in the chain. By the time it gets to
our plate, seafood can have enough toxin to cause serious illness, and even
death. The elderly and those with poor renal function are especially vulnerable
to these toxins (3).

There are
five illnesses caused by harmful algal blooms. One of these, amnesic shellfish
poisoning, is caused by the diatom toxin domoic acid. There has only been one major
human outbreak of ASP to date. In the fall of 1987, over 100 people on Prince
Edward Island in Canada were affected, including three fatalities, after the
ingestion of blue mussells infected with Pseudonitzschia
multiseries (3, 4). They suffered from gastrointestinal and neurological
symptoms including seizures and permanent short-term memory loss. Domoic acid
excites glutamate receptors in the brain, causing neurons to load with Ca2+ and die, causing lesions mostly in the hippocampus, which is responsible for
memory (4). It is under investigation as to whether long-term, low-level
exposure is dangerous (5). Although domoic acid has been used in Japan for
years to treat nematode and roundworm infections without adverse effect, the
amount used is much less than that ingested by consumption of contaminated
seafood (4).

In addition
to the human outbreak in Canada, fish kills and several well documented
incidents of widespread animal death have been observed. Cormorants and brown
pelicans in Monteray Bay in California were poisoned with domoic acid in 1991
(6). They were found to have been feeding on anchovies contaminated with domoic
acid from P. australis (7). Later that year, domoic acid was detected in clams and crabs along the entire West coast of the U.S. (6). In 1996, hundreds of brown pelicans were again affected, this time in Cabo San Lucas in Mexico (7). In the summer of 1998, a population of sealions was affected in the same area as the 1991 seabird outbreak in California. Four hundred sealions died, and many more showed neurological symptoms. Again, anchovies infected with P.
australis were shown to be responsible. During the outbreak, blue mussels,
like the ones that caused the outbreak in Canada a decade earlier, were shown
not to be infected. This shows that simply monitoring mussel populations for contamination is not sufficient (8). Chemical screening of mussels before sale for consumption has prevented further incidents (3), but what about other seafood?

Clearly,
various organisms can be affected by this toxin. Anchovies were the vector in
two major incidents. They filter feed on plankton, and chemicals which are not
metabolized or excreted accumulate in their flesh, and concentrate even more
further up the food chain. During the 1991 outbreak, domoic acid was detected
not only in the stomach contents, but also in the flesh of anchovies studied (6),
suggesting that domoic acid may bioaccumulate in this manner. Anchovies are
heavily preyed upon by not only seabirds and sealions, but dolphins, and salmon
(6). It is possible that salmon stocks could get contaminated, though it is
also possible that the salmon, as the secondary predator, would be poisoned
itself before it would ever be caught for food. Theoretically anything that
eats plankton, or that eats something that eats plankton, could be contaminated
during a bloom and cause illness.

Instead of
relying solely on mussels for detection, DNA probes have been used to monitor
diatom blooms before they become a problem (6). Another approach may be to
predict, or even prevent, blooms before they happen. Pseudonitzschia have been shown to produce more domoic acid when water pH is high, when starved for silicate, phosphate, or iron (9), and when grown with certain bacteria (10), but these conditions are difficult to control. Factors that influence diatom blooms that we do have control over incude local or regional eutrophication of coastal waters, dispersal of species by human travel and shipping, and the many effects of global warming (3).

Global
warming changes weather and ocean current patterns, a slow rise in sea level
and drop in pH. All of these have direct effects on blooms of algae, including
diatoms. Both harmful diatoms and those that feed the ocean’s life are
affected. In addition to continual monitoring of diatom activity and screening
for seafood contamination, further research is crucial for us to understand how
we influence both the diatoms toxic to us and those that drive life in our
oceans.

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